Recently, my bottom bracket was frozen, so I had to remove the crankset in order to replace it. This was done successfully, but after I put the crankset back on, the front derailleur won't shift down onto the lower chainwheel. The chain is stuck on the larger ring and the only direction it will allow me to shift is up, farther past the highest gear, where there is no ring. (i.e., off the largest chainwheel--there are only 2). I've tried adjusting the limit stops, and the derailleur won't line up with the lower chainwheel.

Before I replaced the bottom bracket, I never really shifted up with the front derailleur--I always used the lower chainwheel and never needed to change the front gears. Now, I can't get it back on the lower chainwheel and I don't know what this means--the cage is bent, possibly, although I can't imagine why. I don't know if I should get a new front derauiller or if there's another problem. The cable tension is fine.

it almost sounds like your replacement bottom bracket is narrower than the original and or it is side to side adjustable and not correct.
make sure your cable is not connected and your shifter is in the relaxed position before you set the low limit screw.
disconnect the cable and see if you can move the derailleur by hand.
post some pics as this may also help

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